Many communication systems currently use authentication and encryption to enhance security of the systems. These communication systems include cellular radiotelephone communication systems, personal communication systems, paging systems, as well as wireline and wireless data networks. A cellular communication system will be described below by way of example; however, it will be appreciated by those skilled in the art that authentication and encryption techniques described can be readily extended to other communication systems.
Turning to the cellular communication systems, these systems typically include subscriber units (such as mobile or portable units) which communicate with a fixed network communication unit via radio frequency (RF) communication links. A typical cellular communication system includes at least one base station and a switching center. The switching center that a subscriber unit accesses may not be his "home" switching center. In this case, the subscriber unit is termed a roaming subscriber unit. The switching center which the subscriber unit accesses (termed the "visited" switching center) must communicate with his "home" switching center via the public switched telephone network (PSTN) or other type of connection, such as a satellite link, to retrieve information about the subscriber unit and provide service billing information back to the "home" switching center.
One responsibility of the fixed network communication unit (such as a switching center, location register or authentication center) is to grant use of the communication system to the subscriber unit after the requesting subscriber unit meets the authentication requirements of the system. In a typical cellular telephone communication system, each subscriber unit is assigned a mobile subscriber identifier (MSI), which uniquely identifies the subscriber unit from other subscriber units. In the European cellular communication system, GSM (Global System for Mobile Communications), one such identifier is the international mobile subscriber identification number (IMSI). In order to protect the privacy of the IMSI, GSM calls for most subscriber-fixed network communications to use a temporary mobile subscriber identifier (TMSI) in lieu of the IMSI. In the U.S., EIA-553 .sctn. 2.3 specifies that each subscriber shall have a mobile identification number (MIN) and a factory set electronic serial number (ESN). For convenience all such and similar identifiers will be referred to by the term MSI below.
Detection of a legitimate subscriber's MSI may be accomplished by RF eavesdropping or by purposeful or inadvertent divulgence of the MSI by the radiotelephone installer. Although the IMSI is more protected than the MIN/ESN combination from inadvertent divulgence, the IMSI remains similarly vulnerable to acquisition during RF eavesdropping. Under either protocol, once the subscriber's MSI is known (stolen), a thief may reprogram another subscriber unit with the stolen MSI causing two or more subscriber units to have the same MSI. While cellular radiotelephone systems have authentication procedures to deny access to subscribers not having legitimate MSIs, most typically lack effective capability for detecting multiple users or to minimize the effect of installer leaked MSIs. As a consequence, legitimate users may be billed for both a thief's use of his MSI as well as their own usage.
Because of this problem with illegitimate subscribers (clones) and other forms of fraudulent access, several authentication protocols have been devised. In GSM the fixed network communication unit initiates the authentication process following receipt of a TMSI from the subscriber by generating and sending a challenge (a random or pseudo-random number or RAND) to the subscriber. The subscriber is required to retrieve at least two enciphering elements from its memory: a predetermined ciphering algorithm (e.g., A38) and the subscriber's authentication secret key (Ki). The subscriber then mixes (enciphers) the RAND and Ki into a signed response (SRES) and transmits this signed response back to the fixed network communication unit. If the received SRES matches with the network generated SRES (using the same algorithm and the subscriber's Ki stored in the network), the subscriber is authenticated for service.
In the USA, the United States Digital Cellular (USDC) and CDMA (Code Division Multiple Access) standards are known as IS-54 and IS-95, with an interworking protocol known as IS-41 (all published by the Electronic Industries Association (EIA), 2001 Eye Street, NW, Washington, DC 20006). These use the same basic authentication protocol utilizing a series of specialized messages which must be passed between the subscriber and a communication unit of the network before system access is granted. However, the IS-54/95 protocols employ a "global challenge" of 32 bits in length, as compared with the 128 bit RAND used in GSM. When this challenge is mixed (or encrypted) together with a shared secret key (the SSD.sub.A), the result is an 18 bit signed response (AUTH.sub.-- R) (contrasting with the 32 bit SRES in GSM). Further processing, using the same algorithm and additional shared secret data (SSD.sub.B) or using a different algorithm, is used to generate the message encryption algorithm key and voice privacy mask.
A fundamental problem with having these significantly different authentication protocols is that there is no effective way to provide for "seamless" roaming for subscribers across air interface boundaries. This means that to provide for dual air interface phones, under known methods the subscriber would be required to additionally maintain dual identifiers (and dual accounts) and secret keys for each of the protocols used in the two systems. Even where the subscriber possessed a smart card or detachable subscriber identity module (SIM) capable of use in handsets of different systems, the user would still be required to maintain dual identifiers and have the equivalent of two SIMs and dual registrations for each smart card.
The concept of universal personal mobility has emerged as an important feature of advanced communications networks. However, such universal personal mobility will only be achieved when a user can be readily authenticated even in visited systems employing authentication protocols differing from those of his home system. Therefore, a need exists for a privacy and authentication technique which can alleviate these problems and allow for user roaming across system boundaries.